The invention relates to an optical unit for generating an optical data signal in conformity with a data pattern, said unit having an input for receiving the data pattern with a modulation period T, a pulsed laser for supplying an optical pulse series having a pulse period n.T and a pulse duration .tau., in which n is an integer, and means for converting said pulse series into an optical data signal in conformity with the data pattern.
The invention also relates to an optical transmission system and a transmitter including such a unit.
An optical unit of the type described in the opening paragraph may be used, for example in the transmitter of optical telecommunication systems. In such systems, a digital electric data signal is converted in the transmitter into an optical pulse pattern by modulating the radiation from a laser in the transmitter in conformity with the data signal to be transmitted. Subsequently, the formed series of optical pulses can be transported via, for example an optical fibre to an optical receiver in which it is converted again into a digital electric signal.
In present-day telecommunication systems the aim is, inter alia an increased data transmission rate. This requires optical data pulse series consisting of short pulses. However, optical pulse series consisting of short pulses are relatively difficult to modulate with data.
One of the possibilities of generating a pulse series consisting of short optical pulses and having a pulse pattern in conformity with a data pattern to be transmitted is what is commonly referred to as gain-switching of the laser. In this method a short current pulse is applied to the laser in the case of a digital "1", whereupon the laser in its turn transmits a short optical pulse. However, the following problem then presents itself. To obtain said short pulses, the modulation of the current through the laser should be such that each optical pulse is generated only by the relaxation oscillation of the laser, rather than by the length of the current pulse. The laser oscillation is determined by the charge carrier density and the photon density in the laser medium. The supply of a data pattern, in other words a succession of digital zeros and ones, to the laser means that different current patterns are applied to the laser. These current patterns cause different charge carrier densities in the laser medium. For example, after a number of digital ones has successively been applied to the laser, the charge carrier density will be different than after a succession of a number of zeros. As a result, the shape of the optical pulses will vary. Not only the shape, but also the instant when the optical pulse is formed will be varied so that time jitter is produced. Time jitter is understood to mean that the pulse position is defined inaccurately with respect to the pulse period.
As a result of the phenomena mentioned above, the pulse pattern of the optical data pulse series thus formed no longer corresponds entirely to the original data pattern.
Another manner of generating an optical pulse series consisting of short optical pulses and having a pulse pattern in conformity with a data pattern to be transmitted is the modulation of a pulse series obtained via mode-locking with the aid of an external modulator driven by the data signal to be transmitted. An example thereof is described in the article: "Monolithic semiconductor soliton transmitter" by P. B. Hansen et al. in OFC '94 Technical Digest, pp. 74-75.
The drawback of the use of such a modulator, for example of the electro-absorptive type, for applying a data signal to an optical pulse series is that the electric voltage required by the modulator for obtaining a sufficiently high extinction ratio is relatively high. The extinction ratio is the ratio between the light intensity of a digital "1" and the light intensity of a digital "0". In order that a clear distinction can be made between a "0" and a "1", this ratio should be sufficiently high. Particularly at high transmission rates, this may lead to problems because the modulation frequency increases in this case, and with an increasing modulation frequency the required extinction ratio is relatively difficult to achieve as a result of non-ideal electric transfer characteristics. Moreover, it is relatively difficult to smooth these transfer characteristics so that no level differences and electrical reflections which, moreover, are relatively difficult to smooth, are produced in the range between the DC level and the maximum modulation frequency. Moreover, such modulators are very expensive and the modulation frequency is limited to 10-20 GHz, which is too low for the desired transmission rates.